Echo is a common problem in telecommunications systems. Echo can be very annoying to telecommunications system users, particularly in telecommunications systems having relatively long transmission delays, as for example satellite telecommunications systems and cellular radio telecommunications systems. Consequently, several methods for suppressing echo have been developed.
In one known type of echo suppression, received signals are first processed to determine whether echo is present. If echo is detected, the received signal is processed to suppress the echo. If no echo is detected, no echo suppression is applied.
In one known form of echo suppression, signals containing echo are replaced by a comfort noise signal to eliminate the echo. The comfort noise may be generated by a variety of known techniques.
In one known method for generating comfort noise from an LPC-encoded signal, internal gain and synthesis filter coefficients for a short interval of background noise are used with a pseudorandom excitation provided by a pseudorandom excitation generator or a noise excitation codebook. See, for example, U.S. Pat. No. 5,475,712 entitled Voice Coding Communication System and Apparatus Therefor issued in the name of S. Sasaki on Dec. 12, 1995, and U.S. Pat. No. 5,537,509 entitled Comfort Noise Generation of Digital Communication Systems issued in the names of K. Swaminathan et al on Jul. 16, 1996. Unfortunately, these methods of generating comfort noise provide a comfort noise which sounds somewhat unnatural and which may be discontinuous with background noise present in parts of the speech signal for which no echo is detected. This can lead to distinguishable and annoying discontinuities in the perception of background noise by the user of the telecommunications system.
Consequently, there is a need for a comfort noise generator which provides a more natural and continuous comfort noise for use in echo suppression applications and the like.
U.S. patent application Ser. No. 08/431,224 entitled Methods and Apparatus for distinguishing Speech Intervals from Noise Intervals in Speech Signals and filed in the names of C. C. Chu et al on Apr. 28, 1995 discloses LPC speech encoders and decoders in which LPC coefficients of non-speech frames are averaged over a plurality of consecutive frames during encoding or decoding operations to provide background noise which sounds more natural. Low pass filtering of the resulting signal is also added to make the background noise sound even more natural.